The present invention relates to lifting surfaces such as fixed wings of airplanes and rotor blades for helicopters and other rotorcraft. The invention relates more particularly to a device and method for influencing behavior of the lifting surface using a variable-geometry tip member.
The phenomenon of the tip vortex generated at the free tip end of an aerodynamic lifting surface such as a wing or rotor blade has been the subject of much study. The tip vortex results from spanwise components of flow at the tip end induced by the pressure differential between the suction-side and pressure-side surfaces of the lifting surface. Thus, on the pressure-side surface at the tip end there is an outward spanwise component of flow that tends to cause the flow to roll over the tip end of the lifting surface toward the suction-side surface. As a result, the flow trailing from the tip end of the lifting surface tends to xe2x80x9croll upxe2x80x9d in a vortex. Various undesirable effects accompany the tip vortex. For instance, the tip vortex can negatively impact other bodies behind the lifting surface, such as aircraft following another aircraft from which the tip vortex trails. Additionally, for some operating conditions in a rotor of a helicopter, such as during hover, a rotor blade will encounter a tip vortex generated by a preceding rotor blade. The blade-vortex interaction (BVI) that results is a source of noise and vibration. The tip vortex also causes an induced drag that reduces the aerodynamic efficiency of the lifting surface.
The lift at the outboard end of a wing or rotor blade must reduce to zero at the very tip. This phenomenon results in a reduction in the overall lift that otherwise could be achieved.
Accordingly, considerable effort has been devoted in the aeronautical community to reduce the negative effects associated with the local tip effects of lifting surfaces. One approach for fixed wings has been to attach a xe2x80x9cwingletxe2x80x9d to the tip of a wing. The winglet extends generally normal to the spanwise direction of the wing, either projecting above the suction surface or below the pressure surface. The winglet allows the very tip end of the wing to support a pressure differential such that the lift does not fall to zero. Thus, a winglet effectively extends the span of the wing without actually increasing the span. While the winglet is beneficial in terms of improving the maximum lift coefficient of the wing, this function is needed only during takeoff and landing, since the lift coefficient during cruise is much lower than during takeoff and landing. During cruise, the winglet actually degrades performance because of the increased drag associated with the winglet, and also hurts the maneuverability of the aircraft. Another drawback of the conventional winglet design is that it cannot be applied to rotor blades because the centrifugal forces exerted on the winglet would be too large to be supported by the winglet structure.
Various approaches have been proposed for reducing BVI noise and vibration. One approach is the Ogee tip design used in some helicopter rotor blades. The Ogee design allows the spanwise flow from the pressure-side surface of the blade to roll up in a bigger spanwise region of the blade, thereby attempting to spread the tip vortex out over a larger area and thus reduce its peak strength. As with the winglet, however, the Ogee tip is a passive design that is present at all times, even though it""s beneficial effects may only be needed at certain selected flight conditions, and it can degrade performance at other flight conditions.
Active control devices have been proposed for helicopter rotor blade tips for various purposes. One technique is tip air injection, in which a high-energy jet of air is injected from the blade tip toward the center of the core of the tip vortex in an attempt to weaken the strength of the vortex. Although it can be effective, the system is complicated and heavy, and can degrade the aerodynamic performance to a greater extent than the benefit afforded by the system.
Another active tip device that has been proposed for reducing rotor blade vibration comprises a variable-pitch tip, known as a Smart Active Blade Tip (SABT), that pivots about a spanwise axis, driven by a piezoelectric-induced bending-torsion coupled beam that runs down the length of the blade. The pitching blade tip is intended to create unsteady air loads and, when correctly phased relative to the rotor blade rotation, these unsteady air loads ostensibly can cancel or at least reduce the original unsteady blade loads that contribute to vibration. Although this system may reduce vibration, it does not address blade noise.
Yet another active tip device, proposed for reducing rotor blade noise, comprises a device that droops from the leading edge and is driven by a piezoelectric stack actuator. The objective of this device is to be able to use thinner airfoil sections near the tip of the rotor blade without inducing flow separation on the retreating side of the blade. The device is reported to reduce thickness noise and HSI noise by significant amounts without compromising other aspects of the rotor design. However, the device does not address blade vibration.
The present invention provides an active tip device for a rotor blade that affects the aerodynamics at the blade tip so as to beneficially impact both blade noise and vibration. The device is also useful on fixed lifting members such as wings. Although the device can have a very small spanwise extent as a percent of blade or wing span, model test results suggest that the device can significantly increase maximum lift coefficient while reducing drag coefficient at high angles of attack. An advantage of the device is that if its function is not needed for certain operating conditions, the device can be positioned such that it forms a substantially continuous extension of the outboard end of the lifting member on which it is mounted, thus having minimal impact on the aerodynamics at the tip.
In accordance with one aspect of the invention, a tip member is movably mounted at the outboard end of the main lifting member. The main lifting member at least at the outboard end has an internal cavity defined therein between pressure-side and suction-side surfaces of the main lifting member. The tip member is movable relative to the main lifting member between a neutral position in which the tip member presents substantially no obstacle to a spanwise component of flow over either suction-side or pressure-side surfaces at the outboard end of the main lifting member, and a position in which a portion of the tip member extends beyond one of the pressure-side and suction-side surfaces of the main lifting member so as to present an obstacle to the spanwise component of flow over said surface at the outboard end of the main lifting member and thereby locally affect the flow at the outboard end. An actuation system for the tip member is disposed in the cavity of the main lifting member and coupled with the tip member for moving the tip member so as to vary a distance by which the tip member extends beyond said surface of the main lifting member.
In one embodiment of the invention, the tip member is slidably connected to the main lifting member so as to be slidable back and forth along a direction generally corresponding to a lift direction of the main lifting member. The tip member preferably is configured as a spanwise section of the main lifting member such that when the tip member is in the neutral position there is a substantially continuous and stepless transition between a pressure-side surface of the tip member and the pressure-side surface of the main lifting member and between a suction-side surface of the tip member and the suction-side surface of the main lifting member. When moved out of the neutral position, the tip member creates an inboard-facing step on one surface and an outboard-facing step on the other surface of the main lifting device.
The tip member preferably is affixed to guidance members that extend in an inboard direction from the tip member into the cavity and slidably connect to support members in the cavity affixed to the main lifting member. The guidance members and support members cooperate to substantially constrain movement of the tip member to a translation generally along the lift direction of the main lifting member.
In another embodiment of the invention, the tip member is pivotally connected to the main lifting member so as to pivot about an axis that extends generally in a chord direction of the main lifting member, i.e., in a direction from leading-edge to trailing-edge. The tip member is thus unlike a variable-pitch tip or a variable flap device, each of which pivots about a spanwise axis. The tip member preferably is affixed to guidance members that extend in an inboard direction from the tip member into the cavity and pivotally connect to support members in the cavity affixed to the main lifting member, the guidance members and support members cooperating to substantially constrain movement of the tip member to a rotation about the pivot axis.
Either the sliding or pivoting tip member can be used on a rotor blade for a helicopter or the like, and can be cyclically moved for achieving various effects on the aircraft. To this end, the actuation system in one embodiment includes an actuator for moving the tip member, a sensor mounted in the aircraft for sensing a condition associated therewith, and a controller coupled with the sensor and with the actuator so as to form a real-time adaptive control system for influencing the condition associated with the aircraft. For example, the sensor can be operable to sense a vibration of the aircraft and the controller can be operable to adaptively control movement of the tip member so as to suppress a level of the vibration sensed by the sensor. More particularly, the controller and actuator are operable to cyclically move the tip member with a frequency and amplitude controlled by the controller. An analysis can be performed on the measured vibration so as to derive a plurality of harmonic components of the vibration, and the tip member movement can be controlled so as to reduce at least one dominant harmonic component.
Alternatively, the sensor can measure noise and the controller and actuator can control movement of the tip member to suppress the noise. An analysis can be performed on the measured noise to deduce harmonic components of the overall noise, and the tip member can be moved in a fashion to reduce one or more components of the noise.
It is also possible on rotor blades or fixed wings to control position of the tip member to influence lift of the lifting member. The tip member can either be placed in a stationary position, or its position can be cyclically varied. Furthermore, on a wing the tip member can be cyclically moved so as to impart vibration to the wing and thereby influence behavior of the boundary layer on the wing.